1. Field of the Invention
The present invention relates to a liquid crystal display device and, particularly, to a liquid crystal display device for use as a light valve of a liquid crystal projector.
2. Description of the Prior Art
A liquid crystal display device has a basic construction including a pixel substrate, an opposed substrate opposing to the pixel substrate and a liquid crystal layer sealed in between the pixel substrate and the opposed substrate. On the pixel substrate, a plurality of switching elements such as TFTs (Thin Film Transistors) and a plurality of pixel electrodes, etc., are formed, respectively. A common electrode is formed on the opposed substrate and the opposed substrate is arranged in an opposing relation to the pixel substrate with a predetermined gap therebetween.
FIG. 1 is a plan view of one of pixels of a pixel substrate used in a conventional liquid crystal display device and associated components arranged around the pixel.
As shown in FIG. 1, in the conventional liquid crystal display device, each pixel TFT (channel region 201) is provided at a cross point of a gate line 61 and a data line 81. The channel regions 201 of the pixel TFTs are arranged vertically in the drawing sheet along the data lines 81. Further, in the conventional liquid crystal display device, substantially rectangular storage capacitors 204 are arranged along the gate lines 61.
A portion of the pixel TFT, which has highest light sensitivity, is LDD (Lightly Doped Drain) regions. That is, in FIG. 1, an LDD region 41 on the side of the data line 81 connected to a contact 202 and an LDD region 42 on the side of the data line 81 connected to a contact 204 of a pixel electrode are most sensitive to light.
When an amount of light incident on the data line side LDD region 41 is different from that incident on the pixel electrode side LDD region 42, there is a difference in leakage current between the LDD regions. Besides, since the liquid crystal display device is usually AC driven, electric fields of the data line side LDD region 41 and the pixel electrode side LDD region 42 become high alternately in every frame. Therefore, if there is the difference in leakage current between the data line side LDD region 41 and the pixel electrode side LDD region 42, luminance of the pixel becomes different in every frame, causing flickers to be generated on a display screen.
In order to prevent such flickers from occurring, it has been usual in the conventional liquid crystal display device to employ a construction in which the LDD regions 41 and 42 are optically shielded by an upper light shielding layer and a lower light shielding layer, which are provided above and below these regions.
A typical example of the light shielding structure of the conventional liquid crystal display device has lattice type upper and lower light shielding layers provided to cover the data lines, the gate lines and the pixel TFTs to thereby prevent light from irradiating the LDD regions. This light shielding structure is effective in a case where light fallen on the liquid crystal display device vertically. That is, according to this light shielding structure, the upper and lower light shielding layers can block light entering to the data line side LDD region and the pixel line side LDD region so long as light enters into the liquid crystal display device vertically.
However, light incident on the liquid crystal display device contains not only vertical components with respect to the liquid crystal display device but also various components having a certain incident angular distribution. Therefore, light incident on end portions of the upper light shielding layer may contain lights which are reflected by the lower light shielding layer, etc., repeatedly and directed to the LDD regions. Since, in the light shielding structure of the conventional liquid crystal display device, the amount of light directed to the data line side LDD region and the vicinity thereof is different from that directed to the pixel electrode side LDD region and the vicinity thereof, there is a problem that it is impossible to make leakage currents in the data line side LDD region and the pixel side LDD region equal each other. This problem will be described in detail with reference to FIG. 2 and FIG. 3.
FIG. 2 is a cross section taken along a line I—I in FIG. 1, that is, a cross section of a region including the data line side LDD region 41, and FIG. 3 is a cross section taken along a line J—J in FIG. 1, that is, a cross section of a region including the pixel line side LDD region 42.
Referring to FIG. 2, a lower light shielding layer 2 is formed on an upper surface of a glass substrate 1 and a first interlayer film 3 is formed to cover the lower light shielding layer 2. The data line side LDD region 41 is formed on the first interlayer film 3 and a gate insulating film 5 is formed on the data line side LDD region 41. On the gate insulating film 5, a second interlayer film 7 is formed to cover the gate insulating film. The data line 81 is formed on an upper surface of the second interlayer film 7 and a third interlayer film 9 is formed to cover the data line 81. The upper light shielding layer 10 is formed on the third interlayer film. Since the lower light shielding layer 2 and the upper light shielding layer 10 are formed along the data line, the light shielding layer 2 as well as the light shielding layer 10 is formed slightly wider than the data line 81.
On the other hand, referring to FIG. 3, a metal film 6 which is used as the storage capacitor line 62 is formed on a polysilicon layer 4 and a gate insulating film 5 on both sides of the pixel electrode side LDD 42 and the lower light shielding layer 2 and the upper light shielding layer 10 are formed to cover the metal film 6. That is, since the lower light shielding layer 2 and the upper light shielding layer 10 in this region are formed along the gate line 61, side portions of the lower light shielding layer 2 and the upper light shielding layer 10 in this region are extended regardless of the width of the data line 81.
As mentioned above, the width of the lower light shielding layer 2 as well as the upper light shielding layer 10 in the region including the data line side LDD region 41 is different from that in the region including the pixel electrode side LDD region 42. Therefore, there is a difference in amount of incident light between the respective LDD regions 41 and 42.
For example, light incident on the region surrounding the data line side LDD region 41 at a certain angle with respect to a normal line as shown by an arrow in FIG. 2 is directed to the lower light shielding layer 2 in the vicinity of an edge portion of the upper light shielding layer 10. This light is reflected by the lower light shielding layer 2 formed of a metal material, the data line 81 formed of a metal material and the lower light shielding layer 2 again and reaches the data line side LDD region 41. On the contrary, light incident on the vicinity of the pixel electrode side LDD 42 is blocked by the upper light shielding layer 10 and can not reach the lower light shielding layer 2 even when the incident angle of the light is the same as that shown in FIG. 2, as shown in FIG. 3.
As described, in the conventional liquid crystal display device, the light shielding structure in the data line side LDD region 41 is different from that in the pixel electrode side LDD region 42. Therefore, there is a problem that leakage currents in the data line side LDD region and the pixel side LDD region are different. Further, there is another problem that degradation of image quality such as flickers on the display screen occurs due to the difference in the leakage current.
These problems become specifically importance when the liquid crystal display device is used as a light valve of the liquid crystal projector. That is, according to a recent tendency of miniaturization of a projector casing and miniaturization of a liquid crystal panel in order to accommodate to reduction of cost while increasing the luminance of screens, an amount of light incident on a liquid crystal display device is increased, so that even a small difference of leakage current leads to degradation of image quality.